Predictions for the 2004 Transit of Venus

Fred Espenak, NASA/GSFC, and, Jay Anderson, Environment CanadaBased on a Paper Presented at the 2004 Meeting of the American Astronomical Society in Atlanta, GA, 2004 January 4-8

A transit of Venus across the disk of the Sun is
among the rarest of planetary alignments. The last transit occurred
120 years ago in 1882, while the next one takes place on 2004 June
08. Figure 1 (Small Figure,
Large Figure) shows the
geocentric path of Venus across the Sun. The scale along the track
gives the Universal Time[1] of Venus's position at any
instant. The planet moves westward with respect to the Sun and
inscribes a chord through the Sun's southern hemisphere. Moving
with an angular speed of 3.2 arc-minutes per hour, Venus takes
about 6.2 hours to cross the Sun's disk.

The transit begins with contact I when the limb of Venus
is externally tangent with the Sun. It takes about nineteen minutes
for the planet's disk to cross the solar limb where it becomes
internally tangent with the Sun at contact II. The period
between contacts I and II is known as ingress. At the end of
ingress, the entire disk of Venus is seen silhouetted against the
Sun's disk as it begins its slow six-hour passage across our
star.

Contact III occurs when Venus reaches and first touches
the opposite limb of the Sun. Another nineteen minutes elapse as
the planet gradually exits the solar disk. Finally, the transit
ends with contact IV when Venus's disk completely exits the
Sun and the planet vanishes from sight. The period from contact III
to IV is referred to as egress.

Table 1

Geocentric Phases of the 2004 Transit of Venus

Event

Universal Time

Description

Contact I

05:13:29

ingress
begins

Contact
II

05:32:55

ingress
ends

Greatest

08:19:44

minimum angular
distance

Contact
III

11:06:33

egress
begins

Contact
IV

11:25:59

egress
ends

Table 1 gives the times of major events during the transit.
Greatest transit is the instant when Venus passes closest to the
Sun's center (i.e. - minimum angular separation). During the 2004
transit, Venus's minimum separation from the Sun is 627
arc-seconds. These contact times are actually for an observer at
Earth's center. The contact times at other locations will differ up
to plus or minus seven minutes. This effect is due to parallax
since Venus's position will shift slightly[2] depending on
the observer's geographic coordinates on Earth. For the times of
each stage of the transit from various cities around the world, see
the tables at Transit Contact
Times.

[1] Universal Time or UT is the basis for the worldwide
system of civil time. It is often referred to as Greenwich Mean
Time although UT is actually based on atomic clock time rather than
the Sun's mean motion. For more information, see All About Time Zones
and Universal Time.

[2] Venus's 58 arc-second disk may be shifted up to 30
arc-seconds from its geocentric coordinates depending on the
observer's geographic position on Earth.

The transit will be visible from the entire daylight hemisphere
of Earth. But since the event lasts over six hours, Earth will
rotate 1/4 revolution between the times of ingress and egress.
Consequently, some geographic areas will see the entire transit
while others witness only part of the event (including either
ingress or egress). Finally, about a quarter of Earth will see none
of the transit since the Sun will be below the horizon throughout
the entire period.

At ingress (Figure 2 - Small Size, Large Size), the transit will
be observable from all of Asia, Australia, Europe, and high
northern latitude including Alaska, northern Canada, Greenland and
Iceland. At greatest or mid-transit (Figure 3 - Small Size, Large Size), the event will be
visible from Europe, Africa, Asia, western Australia, and high
northern latitudes. At egress (Figure 4 - Small Size, Large Size), the transit will
be observable from all of Africa, Europe, central Asia, eastern
North America, northern South America, and high northern latitudes
including Greenland, Iceland and northern Canada.

As Earth rotates, the Sun will set before the transit ends from
Japan, Indonesia, the Philippines, Australia and easternmost Asia,
so these regions will miss egress. Similarly, locations in western
Africa, eastern North America, the Caribbean and northern South
America will find the transit already in progress as the Sun rises.
As a result, these locales will miss ingress.

The entire transit is observable from Europe, north and east
Africa, and Asia (except far east). In contrast, none of the
transit will be seen from western North America, the eastern
Pacific (including Hawaii), southern South America (Chile and
southern Argentina) and Antarctica. Western states in the U. S.
which miss the transit include: Arizona, California, Colorado,
Idaho, Nevada, New Mexico, Montana, Oregon, Texas, Utah, Washington
and Wyoming.

The world visibility map (Figure 5 - Small Size, Large Size) depicts the
geographic locations from which each phase of the transit is
visible. This map does not include refraction, which could increase
the region of visibility by about half a degree.

The following tables provide contact times and
corresponding altitudes of the Sun for hundreds of cities and
locations throughout the world. For convenience, the times of
sunrise and sunset for each city are also provided. Greatest
transit is the instant when Venus passes closest to the Sun's
center (i.e. - minimum angular separation). Please note that all
times listed are in Universal Time (UT). To convert from Universal
Time to local time, you must know your Time Zone and whether or not
you are on Daylight Savings (sometimes called Summer
Time). For more information, see All About Time Zones and Universal Time.

Both New Zealand and the Hawaiian Islands lie at the extreme edges
of the zones of transit visibility. Consequently, they are not
included in the above tables because the observations will be
challenging and require an unobstructed horizon along with perfect
weather. Nevertheless, a number of people have requested
information about the transit from these extreme locations.

The beginning of the transit may be visible from the very
northernmost parts of New Zealand only minutes before sunset. From
Auckland, external ingress (contact I) takes place at 05:06:42 UT.
The Sun's altitude will be about 0.4 degrees (includes refraction).
The Sun will set a few minutes later (about 05:10 UT).

Please note that from the Hawaiian Islands, the transit begins
just before sunset on June 7!

From Kauai, External Ingress (contact I) takes place at 05:08:52
UT (07:08:52 pm). The Sun's altitude will be about 1.7 degrees
(includes refraction). The Sun will set about ten minutes later
(07:19 pm). From Oahu, External Ingress (contact I) also takes
place at 05:08:52 UT (07:08:52 pm) but the Sun's altitude will only
be about 0.4 degrees (includes refraction). The Sun will set about
five minutes later (07:12 pm). The transit is not visible from the
Big Island of Hawaii.

The rarity of this event demands that special attention be given
to the long-range weather prospects. Cloud cover maps (Figure 6 -
Small Size, Large Size) show that the
most promising skies for transit observations occur in two parallel
low latitude bands straddling the equator. South of the equator,
this band of minimal cloudiness runs from Brazil to southern Africa
and Australia. In the northern hemisphere, the band runs across the
southern United States, northern Africa and the Middle East, being
interrupted across India and Southeast Asia by the onset of the
southeast monsoon. The two regions are separated by the high
cloudiness associated with the Intertropical Convergence Zone (ICZ)
that runs approximately along the equator.

The low cloud zones are associated with the semi-permanent high
pressure anticyclones found in June over the oceans at 35°N and
at 25°S. These sub-tropical anticyclones are formed by the
large scale Hadley circulation in the atmosphere that exchanges
heat and humidity in equatorial regions with colder and drier air
near the poles. Descending air in the center of these anticyclones
dries the atmosphere through adiabatic warming and it is no
accident that the world's great deserts are also found at these
latitudes - among them, the Sahara, Kalahari, Atacama, and Arabian
Deserts.

Away from the tropics, cloud cover increases rapidly with
latitude, though the pattern is greatly modified by the presence of
mountain chains that interrupt the flow of weather systems and the
transport of moisture from oceans onto land. In North America, the
Rocky Mountains block the flow from the Pacific into the continent
and permit a zone of low cloudiness that stretches northward from
Texas to the Canadian border. In northern Europe, Atlantic moisture
flows unrestricted onto the land until being blocked by the Alps
and Pyrenees, keeping Germany, France and England under high levels
of cloudiness, while preserving the sunnier climates of Italy and
Greece. Similar effects can be seen in southern Asia around
Kazakhstan and Mongolia, although the favorable cloud patterns
there are due as much to the absence of nearby moisture sources as
the presence of mountains.

For those who are determined to see the transit in its entirety,
the most favorable climatology lies in regions surrounding the
Mediterranean, the Middle East, and portions of southern Africa.
The Azores High, the closest anticyclone, extends a tentacle of
high pressure down the length of the Mediterranean, suppressing
cloud-bearing weather systems and helping to extend the reach of
the Sahara's dry influence into Italy, Greece and the coasts of
Turkey and Spain. Sunshine prospects are even better on the African
coast. Clear skies are virtually guaranteed, but high temperatures
may make observation of the transit a test of endurance at inland
sites.

North America's deserts lie outside the transit's view, but the
western plains from Oklahoma northward, offer the best chance for a
tantalizing glimpse of the end of the transit at sunrise. The
Appalachians and Florida are best avoided unless the forecast of
the day calls for sunshine. The entire transit can be seen in the
far north, where the sun maintains a continuous 24-hour presence,
but cloud prospects border on awful. The global cloud map shows a
minimum in cloudiness through parts of Alaska and Canada's Yukon
Territory where mountains block some of the flow from the Pacific,
but reaching the area will require an expedition down the Alaska
Highway, or a flight to one of several high Arctic communities.

Meteorological statistics for the frequency of various amounts
of cloud cover for a number of cities in the U. S., Canada, Europe,
the Mediterranean and the Middle East can be found in the following
tables.

In 1716, Halley proposed that transits of Venus could be used to
measure the Sun's distance, thereby establishing the absolute scale
of the Solar System and solving one of the greatest problems in
astronomy at that time. The technique required that expeditions
travel to the far ends of Earth so that the differing parallax of
the observations could be used to derive the distance to Venus.
Today the distance to the Sun and planets can be measured extremely
accurately using radar, so the 2004 transit will have no scientific
value in this regard. Still, it is a remarkably rare event, which
was of great importance during the early history of astronomy.

With the recent discovery of the first transit of a planet
around another star [Henry, Marcy, Butler, and Vogt, 1999] interest
in extra-solar transits is high. The 2004 transit of Venus may be
of use in developing and testing new techniques and strategies for
the detection and characterization of other extra-solar
planets.

However, the greatest value of the 2004 transit lies in public
outreach and educational opportunities to share this unique event
with non-scientists. The public, amateur astronomers, educators,
students and the media are genuinely fascinated with the transit
and its rich scientific, cultural, political and intellectual
history. A few of the educational Web sites focusing on the transit
include:

http://sunearthday.nasa.gov/ - NASA Goddard's Sun-Earth Connection Education Forum has selected
the transit as its theme for 2004. It has organized a series of
resources and activities for every classroom and age group. It also
live webcasts of the transit.

Rarely does an astronomical event occur which can be observed
directly by the public using simple equipment. With justifiable
concerns about the scientific literacy of today's citizens, the
transit is a golden opportunity to stimulate, educate and (dare we
say) entertain. It may even serve to ignite the imaginations of a
few students who may continue on to become part of the next
generation of scientists.

The predictions presented at this NASA 2004 Transit of Venus web
site are provided as a resource for the public, educators, media
and especially students attempting to measure the scale of the
solar system.